Machine vision inspection systems detect errors on printed circuit boards that occurred during the process of applying solder paste onto the bare boards. Machine vision inspection systems also detect geometry and placement errors that occurred during the process of automatically or manually populating the circuit boards with component parts. Such errors, theoretically perceptible by the human eye, include whether leads of discrete through-hole technology components have been correctly inserted through the proper solder paste pads on the circuit board. Other errors include the presence or absence of components, translational or rotational errors in the placement of the SMDs relative to the pad, and whether the SMDs were placed on their respective pads on the correct face. Machine vision inspection systems also measure the lead length and wipe angle of through-hole technology components and the various geometric characteristics of SMD technology components.
Machine vision inspection systems typically employ video cameras to capture visible spectrum images of the subject printed circuit boards. The video cameras are mounted on either a fixed or movable staging structure above an automated conveyor or a printed circuit board shuttle. The video cameras capture images of the printed circuit boards as they pass under the camera staging apparatus on the conveyor or shuttle. The captured images are first digitized, then compared to a master database of ideal art work to determine whether any of the above-mentioned geometry or placement errors are present.
Adequate and properly directed illumination of the screened solder paste and/or the through-hole and SMD components mounted on the subject circuit boards is critical to gathering accurate input data for the computer algorithm that correlates the digitized video camera image with the ideal art work. Adequate and properly directed illumination is also critical to the detection of all the errors present on each circuit board.
In the past, light has been delivered "straight-on" to the circuit board components within the video cameras' field of view from four discrete xenon lamps that were positioned parallel to the sides of the printed circuit board, as illustrated in FIG. 4 and described below. This straight-on approach proves disadvantageous because too much space is required to accommodate the four discrete light sources. More importantly, the straight-on approach fails to achieve shadow-free illumination of all four sides and the top of the essentially cubical SMD technology components. The straight-on approach fails to achieve shadow-free illumination because, except for the few components centered within the light sources, each xenon lamp is necessarily closer to some components and farther away from others disadvantageously resulting in shadows and bright spots across the printed circuit board.